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Full text of "Text Book Of Mechanical Engineering"

620

Indicated Horse Power.

serious cylinder condensation. The upper and lower diagrams at j
are from the top and bottom of the piston respectively in a
Cornish single-acting pumping engine : and M shews the varying
diagrams obtained from a locomotive, (i) when starting, next (2),
and lastly (3), as the valve gear is linked up from the reversing
lever.

From the Indicator diagram we may therefore deduce :

1.  The points of admission, cut off, release, compression, &c.

2.  Comparison of cylinder with boiler pressure.

3.  The wire drawing in steam and exhaust passages.

4.  The back pressure.

5.  The condensation, re-evaporation, and relative dryness.j

6.  The indicated horse power from the diagram area.

Calculation of Indicated Horse Power, or that shewn
upon the indicator diagram, and representing the work given to
the piston by the steam or gas.* Three pairs of diagrams, in
Fig. 624, are taken from the respective cylinders of a triple-
expansion engine; and are copied from the Hons. Engineering
Exam. 1887. The mean effective pressure per square inch (p) must
first be found, so the diagrams are divided into 10 parts by
equidistant vertical lines. Knowing the scale of the indicator
spring, the pressure may be measured at the middle of each
division, within the enclosed curve; these figures representing the
effective pressures. Notice that at A and F, Fig. 623, the loop
encloses minus effective pressure; every measurement must there
be treated as minus, and only added to the other plus measure-
ments algebraically. Adding the 10 measured parts, and dividing
by 10 gives mean effective pressure for each diagram; the mean
of the pair being then found by adding them and dividing
by 2.

Multiplying (p) by piston area (a) gives total mean pressure,
and this" again by stroke in feet (L) gives work in foot pounds per
stroke. Further multiplying by number of strokes per minute (N)
gives work per minute, and the whole divided by 33,000, or one

* Brake  horse   power   is   found by dynamometer,  as at p. 575,  and
.echanicaLefficiency of engine 

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